Breaking Change: Do not perform two-way data binding by default

[alexlafroscia]

Two-way data binding is an anti-pattern in Ember applications these days, and should be avoided where possible.

Currently, ember-pikaday performs two-day data binding to the value property by default. This behavior would be removed in favor of one-day data binding and better documentation of how to use the mut helper to explicitly bind to changes using onSelection

<PikedayInput
  @value={{value}}
  @onSelection={{action (mut value)}}
/>